Avionics displays provide critical flight information to aircraft pilots. It is expected that such displays are readable under a variety of lighting conditions. At one extreme, displays must be readable in fall daylight conditions as well as at the other extreme, in complete darkness. Sudden changes in the interior cockpit lighting conditions may occur, such as when the general cockpit lighting is turned on or off or when clouds block direct sunlight. An appropriate amount of backlight illumination is required to ensure consistent, readable avionics displays under a variety of changing lighting conditions.
Providing an appropriate amount of backlight requires a broad range of illumination. In dark ambient light conditions, low levels of backlight may be appropriate, such as 0.1 fL (foot Lamberts), whereas as in bright ambient light conditions, greater levels of light generation, such as 200 fL, are appropriate. Once the appropriate light level is determined, various factors may impact the amount of light actually generated.
One factor is temperature of the electrical components. Temperature variations of components can be caused by ambient cockpit temperature changes or heat generated during use of the electrical components. Backlight control units should compensate for changes in light levels due to temperature variations.
Age of the components is another factor impacting the amount of light generated by the backlight. Electrical characteristics of components gradually change over time, and consequently, the light produced by a backlight may gradually change. Backlight control units should account for changes in light levels due to age of the components.
In the past, fluorescent bulbs have been used to provide backlight to avionics displays along with various control units for dimming fluorescent bulbs. Such systems are disclosed in Patent Application U.S. Pat. Nos. 5,296,783 and 5,428,265. However, use of fluorescent bulbs for dimmable backlighting presents several undesirable characteristics. First, fluorescent bulbs have a finite life and are prone to sudden failures. The failure of a single bulb may render the display unreadable and replacing bulbs constitutes an unscheduled maintenance action which can adversely impact flight schedules. In addition, fluorescent bulbs are particularly temperature sensitive with regard to light generation as a function of their operating temperature, with a warm fluorescent bulb generating more light than the same bulb colder. Finally, fluorescent bulbs require high alternating voltage levels for operation. This is undesirable for several reasons, a few of which are as follows. First, a high voltage requires a dedicated high voltage power source adding to the complexity and weight of the airplane. Second, high voltages increase the risk of sparks due to malfunctions, such as a short circuit, presenting a potential danger. Third, electrical circuitry controlling high voltage is prone to high frequency signal generation (i.e., electrical ‘noise’) which can interfere with the operation of other electrical aircraft systems.
Thus, there is a need for a flexible control unit providing a wide dimming range of light generated in a backlight for an avionics display without requiring high voltages, providing reliable light generation, and that is less sensitive to temperature changes.